If any of the above commands fail, troubleshoot further on the Windows side, depending on the error message returned.

If they are successful: -

3) check connection logs on the NAS device to ensure that the connection was authenticated as the "agent credential" user when the above tests were done (if the share is on a Windows server, this information will be in the Security event log). If not, there may be credentials cached in the Windows desktop session.

4) try to run another backup, and check the connection logs again to find out if authentication using the agent credential has failed